Improvements in the image qualities of color photographic materials, including their color reproducibility, sharpness and graininess, are noticeable in these days. However, there is no end to the user's demand for higher image qualities of photographic materials, and further improvements in photographic materials are desired.
Given the situations, various techniques for producing emulsions and couplers have been studied, obtaining excellent results.
However, many of these techniques often worsen the stability of photographic materials during storage. In particular, they often increase the fog of photographic materials or increase the latensification thereof during storage. Therefore, the technique for overcoming the problems has been desired.
Picture-taking photographic materials are developed, after having been exposed. The exposed materials may be developed immediately, but as the case may be, they are developed in several months or even in one year or more after exposure. Therefore, it is desired that the properties of the exposed photographic materials do not change with the lapse of time during storage. It has been known from the past that the stability of latent images in exposed photographic materials depends on the latent image fading which is seemingly caused by the decrease in the sensitivity of the materials and the latensification which, on the contrary, is seemingly caused by the increase in the sensitivity thereof. Recently, for example, there are known studies by E. F. Thurston in The Journal of Photographic Science, Vol. 38, pp. 34 to 40, 1990, in this regard.
It is well known in this technical field that the technique for increasing the sensitivity of silver halide emulsions for employing fine silver halide grains is the most important so as to improve the graininess of photographic materials. In order to increase the sensitivity of fine silver halide grains, investigation of reduction sensitization methods have been made in these days (for example, see JP-B 57-33572 (corresponding to U.S. Pat. No. 3,892,574), 58-1410 , JP-A 57-82831 (corresponding to U.S. Pat. No. 4,401,754), 57-179835, 2-136852 (corresponding to U.S. Pat. No. 5,254,456);--the terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined and published Japanese patent application", respectively), and these have obtained great results. However, it has been found that the increase of the sensitivity of photographic materials by reduction sensitization involves the increase of fogging during storage and the increase of latensification.
On the other hand, it has heretofore been known that the fog of photographic materials may be retarded and the desilverability thereof may be improved by incorporating polymers having repeating units derived from vinylpyrrolidone compounds (for example, see U.S. Pat. No. 3,505,067). However, the technique was still unsatisfactory for sufficiently inhibiting of fogging during storage and it was quite ineffective in retarding latensification.
JP-A-1-210948 has disclosed a technique of improving the image quality of photographic materials and of inhibiting fogging during atrage by employing polymers having repeating units derived from vinylpyrrolidone compounds and having a mean molecular weight of 30,000 or less and compounds that react with an oxidation product of a color developing agent and provide no image density.
By the present inventor's experiments, however, it has been found that the improvement in the inhibition of fogging during storage according on the method disclosed in JP-A-1-210948 is insufficient and that latensification is not retarded. It has rather been found that the compounds which react with an oxidation product of a color developing agent and provide no image density often increase latensification, and that the compounds have such an unfavorable side effect that they lower the sensitivity of photographic materials containing them.